1. Field of the Invention
The present invention relates to a method for preventing and/or controlling non-microbial staining in the sapwood of lumber, to an apparatus for carrying out the invention method, and to non-staining lumber, preferably non-staining lumber obtained by the invention method. The invention method applies mechanical force, preferably compression and/or vibration force, to the surface of freshly-cut lumber to prevent and/or reduce non-microbial sapwood staining. This non-chemical control method poses no threat to the environment, generates no hazardous waste and is easily incorporated into the green chain of existing lumber mills.
2. Discussion of the Background
Perhaps the major concern of the saw mill industry today is the production of lumber having the best possible appearance. Due to the inherent nature of wood, however, sapwood tends to discolor because of microbial and non-microbial staining. The staining leads to a loss in aesthetic quality and results in the degradation (both physical degradation and "degrading" as that term is used in the lumber industry to denote a lowering in wood quality grade and commercial value) of lumber from many tree species. Red Oak (Quercus spp.) and Ash (Fraxinus spp.) suffer the largest monetary losses due to staining, but losses also occur in other hardwood and softwood species. In fact, it currently is estimated that in the state of Mississippi alone, non-microbial enzymatic staining costs mill owners approximately twenty million dollars annually.
While it has been determined that microbial staining can be controlled through proper drying techniques and through the use of biocides (i.e., dip treatment of unseasoned lumber, or the injection of biocides into wood under pressure to protect against decay from fungus and/or insects and producing "pressure-treated lumber"), non-microbial enzymatic staining has not been widely addressed. Recently, however, one of the present inventors (Amburgey) and Schmidt determined that non-microbial staining in sapwood is caused by enzymes and/or chemical precursors of stain compounds produced by parenchyma cells present in the wood. See U.S. Ser. No. 08/056,685; University of Minnesota Docket No. 93065, filed Apr. 30, 1993 incorporated herein by reference.
Parenchyma cells in wood release enzymes and/or chemical precursors of wood stain compounds which eventually provide a discoloration of the sapwood. Often, the staining is non-uniform, mottled and has a gray appearance on the flat-sawn surface, making the wood unsuitable for many applications and causing the lumber to be assigned a low grade and to be sold for a lower price. Such non-microbial enzymatic staining tends to discolor the entire sapwood when observed in transverse section, as opposed to microbial (fungal) staining which forms wedge-shaped patches along rays. In the beginning stages, non-microbial enzymatic staining often appears at the heartwood-sapwood interface and later progresses throughout the available sapwood. Generally, the stain is not noticeable in rough-sawn lumber until the outer surface is removed by planing or sanding or a fresh cross-section is exposed by end trimming.
The prevention of non-microbial enzymatic staining has been accomplished chemically. For example, the treatment of lumber with sodium bisulfite has prevented the formation of non-microbial enzymatic staining. See Amburgey, T. L., P. Forsyth, 1987. Prevention and control of gray stain in southern red oak sapwood. In: Proc., 15th Annual Hardwood Symposium of the Hardwood Research Council, Memphis, Tenn. May 10-12, 1987, pp. 92-99; Forsyth, P. 1988, Control of non-microbial sapstains in southern red oak, hackberry, and ash lumber during air-seasoning. A thesis submitted to the Faculty of Mississippi State University in partial fulfillment of the requirements for the Degree of Master of Science in the Department of Wood Science and Technology, Mississippi State, Mississippi, pp. 1-50; Forsyth, P. G., T. L. Amburgey, 1991, Microscopic characterization of non-microbial gray sapstain in southern hardwood lumber. Wood and Fiber Sci. 23(3): 376-383; Forsyth, P. G., T. L. Amburgey, 1992, Prevention of non-microbial sapstains in southern hardwoods; Forest Prod. J. 42(3):35-40. The fumigation of logs with methylbromide has similarly prevented the development of non-microbial enzymatic stains. See Schmidt, E. L., T. L. Amburgey, 1994, Prevention of enzyme stain of hardwoods by log fumigation, Forest Prod. J. 44(5):32-34. However, because of the expense in handling, required line production changes and hazards presented by chemical treatment the commercial lumber industry has not adopted these chemically-based treatment processes.